1. Field of the Invention
This invention relates generally to a pressure indicator, wherein pressure is indicated by a change in density. The pressure indicator is utilized in a method for measuring pressure in a high pressure vessel, one example of which is in high hydrostatic pressure processing.
2. Description of the Prior Art
High Hydrostatic Pressure (HHP) processing has been successfully commercialized by several companies around the world on foods such as hams, fruit juices, jams, guacamole and oysters to reduce the risk of microbial contamination and extend shelf life. Isostatic compression transfers pressure instantly and uniformly throughout the pressure medium providing a non-thermal process alternative for the pasteurization of temperature-sensitive foods. Foods suspended in the pressure medium are assumed to follow the isostatic principle.
The unique design of the HHP equipment restricts access to the interior of the pressure vessel during operation thereby preventing direct measurement of the pressure using standard pressure gauges. No methods have been reported for measuring the pressure within the HHP vessel or within a food product during the HHP process.
Effects of HHP processing on the microbiological, physical and chemical aspects of various food systems have been the topic of much research. The pressure within the HHP vessel is currently measured indirectly by gauges measuring the pressure media or the expansion of the yolk on the HHP unit itself.
Processing powdered ceramics and metals using isostatic high-pressure gas or HHP is an established science. Cold Isostatic Pressing (CIP) of powdered metals was first described by Madden in 1913 in a U.S. patent assigned to the Westinghouse Lamp Co. The process consolidates powdered metal or ceramic into a more dense structure that is near the net shape of the desired finished product through the use of isostatic pressing, similar to packing a snowball. This xe2x80x98near net shapexe2x80x99 is referred to as a xe2x80x98green bodyxe2x80x99 since it requires further densification and hardening by sintering.
CIP parts are produced using either a wet bag or dry bag process. As the names imply, a wet bag process uses a pressurized liquid medium to compress a powdered material into a solid shape that is protected from the liquid by an elastic mold. The dry bag process uses fixed molds and is pressurized by gases. Pressures typically used to produce CIP products range from 55 MPa for Teflon powders to 400 MPa for iron and stainless steel powders.
Hot Isostatic Pressing (HIP) was developed by the Battelle Memorial Institute in 1956 to bond nuclear fuel elements. HIP""ing is generally performed at pressures lower than 200 MPa at temperatures ranging from 500xc2x0 C. to 2200xc2x0 C. using argon or nitrogen gases. Combining high-pressure and high-temperature processes eliminates the sintering step associated with the CIP process.
Hite and Bridgeman pioneered research using HHP processing to inactivate bacteria in milk and denature egg albumin proteins in the late 1800xe2x80x2s and early 1900xe2x80x2s. HHP processing of foods has been extensively studied during the last century but equipment technology constraints prevented commercialization of the process until the 1990xe2x80x2s. Pressures as high as 1000 MPa have been studied but equipment limitations limit the practical operating range between 200 MPa and 600 MPa.
The process is governed by Le Chatelier""s principle which states that a system at equilibrium adjusts when subjected to a stress. The principle for using the HHP process as a pasteurization method is based on the assumption that the product also follows the isostatic rule. This rule states that isostatic pressure is instantly and uniformly transmitted throughout the pressurized medium and the enclosed food product, regardless of size, shape or physical state of the food.
The mechanical compression of powders to form tablets for pharmaceuticals, confections and other uses has been studied by a number of authors. The science of powder technology is reviewed quite well by editors Fayed and Otten in the Handbook of Powder Science and Technology, New York, N.Y., Van Nostrand Reinhold Company (1984) and Alderborn and Nystrom in Pharmaceutical Powder Compaction Technology, New York, N.Y., Marcel Dekker, Inc. (1995). Tablets are typically formed by direct compression using a uniaxially oriented force in a punch and die mechanical operation. Table densities are not uniform due to the speed and force of the upper or lower punch, effects of the side-walls of the die, degree of die lubrication and tablet formulation.
The present invention addresses the problems associated with the prior art and provides for an irreversible pressure indicator that may be inserted into a high pressure vessel or food product to determine pressure that was achieved during the process.
In one embodiment, the invention is a method of measuring pressure inside of a high pressure vessel. The method includes compressing a substance at a first pressure to create a preform having a first density. The preform is placed in a high pressure vessel. The preform is then subjected to a second pressure, greater than the first pressure, thereby changing the preform to a second density, greater than the first density, whereby the second pressure can be determined.
In another embodiment, the invention is a method of monitoring pressure inside of a high pressure vessel during food processing. The method includes placing a food product inside of a high pressure vessel. A preform, having a first density, is placed in the high pressure vessel. The food and the preform are subjected to a high pressure in the vessel, thereby changing the density of the preform, whereby the pressure in the vessel can be determined.
In another embodiment, the invention is a method of measuring pressure inside of a food product, while the food product is subjected to high hydrostatic pressure. The method includes placing a food product inside of a high pressure vessel. A preform, having a first density, is positioned in the food product. Then, the food product and preform are subjected to a high pressure in the vessel, thereby changing the density of the preform, whereby the pressure in the food can be determined.
In another embodiment, the invention is a method of forming a solid shaped body. The method includes compressing a face-centered cubic crystal powder at a first pressure to create a preform. The preform is moved to a high pressure vessel. The preform is then subjected to a second pressure, greater than the first pressure, to increase density of the preform, whereby the second pressure can be determined.
In another embodiment, the invention is a kit for determining pressure in a high pressure vessel. The kit includes a preform formed from a plastic material, wherein density changes at pressures greater than 100 MPa are irreversible. The preform is formed under a first pressure, having a first density. Also included are instructions for placing the preform in a high pressure vessel, wherein the preform has a second density, thereby enabling the second pressure to be determined.